1. Field of the Invention
The present invention relates generally to an engine control apparatus for detecting a misfire on the basis of a detected ion current value and correcting engine control parameters upon detection of misfire. More particularly, the present invention is concerned with an engine control apparatus in which the reliability in misfire detection is improved or enhanced by preventing erroneous detection of the ion current level due to superposition of noise.
2. Description of the Related Art
In general, in an internal combustion engine (hereinafter referred to simply as an engine) driven by a plurality of engine cylinders through a crankshaft, a reference position signal generated in synchronism with the rotation of the engine is utilized for determining a variety of timings for the engine operation controls such as an ignition timing, a fuel injection timing and the like. To this end, an angular position detector for generating a reference position signal is mounted on the crankshaft or a camshaft of the engine at such a position that the reference position signal as generated indicates a predetermined reference position,which corresponds to a predetermined crank angle (i.e., angle of rotation of the crankshaft).
When a misfire occurs in an engine cylinder which is to be controlled in the ignition cycle, being accompanied with no explosion, an abnormal explosion known as after-burning will take place after the ignition cycle, involving injury of the engine cylinder and/or damage of a catalyst employed for catalytic treatment of the exhaust gas due to the contact with an uncombusted gas mixture (i.e., air/fuel mixture undergone no combustion). Under the circumstances, a variety of measures are developed and adopted in an attempt for evading occurrence of misfire in the engine, to thereby secure protection of the engine.
For a better understanding of the background of the present invention, an engine control apparatus will be described in some detail by reference to the drawings.
FIG. 5 is a block diagram showing the general arrangement of the engine control apparatus.
Referring to the figure, a reference numeral 1 denotes an angular position detector which is constituted, for example, by a rotatable disk mounted on a camshaft of the engine for generating a pulse-like reference position signal T.theta. corresponding to a predetermined crank angle (reference position) in synchronism with the rotation of the engine. Usually, the reference position is set, for example, at a crank angle or position of 75 degrees (i.e., B75.degree.) or 5 degrees (B5.degree.) before top dead center of each engine cylinder. A set of sensors, collectively designated at reference numeral 2, detect a variety of engine operating parameters such as an intake air flow (or an opening degree of a throttle valve) indicative of an engine load, a rotation speed (rpm) of the engine, an intake air temperature and so forth, and generate corresponding signals D. An ion current detector 20 detects ions generated within the engine cylinder through the explosive combustion. The ion current detector 20 serving for a combustion state detecting function is provided in association with all the engine cylinders or alternatively for a given number of the engine cylinders, respectively, as occasion requires. A control unit in the form of a microcomputer, generally designated at reference numeral 3, includes an engine control parameter setting circuit 31 for arithmetically determining a control parameter Ta for each engine cylinder on the basis of the reference position signal T.theta. and the engine operation state signals D mentioned above, and a misfire detecting circuit 32 for generating a misfire detection signal C on the basis of the reference position signal T.theta. and the detected ion current value I. The misfire detecting circuit 32 has a comparison function for comparing the detected ion current value I with a reference level *which depends on the engine operation state signal D.
The engine control parameter setting circuit 31 is so designed as to generate, as the engine control parameter Ta, a control timing signal which corresponds, for example, to the ignition timing, and at the same time perform a misfire suppression processing (e.g., increasing of electric energy for the ignition coil of the misfired cylinder) on the basis of the misfire detection signal C generated when the detected ion current value I indicates a misfire level or perform control of stopping fuel injection for the misfired cylinder, to thereby suppress discharge of the uncombusted gas mixture. As the engine control parameter Ta, not only the ignition timing but also other various control parameters such as the fuel injection timing, duration of the power supply to an ignition coil for the misfiring cylinder, etc., can be employed.
FIG. 6 is a circuit diagram showing the structure of the ion current detecting detector 20. As can be seen from this figure, the ion current detector 20 comprises an ignition coil 21 having a primary winding 21a and a secondary winding 21b, a power transistor 22 for breaking a primary current i.sub.1 flowing through the primary winding 21a in response to an ignition trigger pulse P generated in an ignition timing sequence, a spark plug 23 for producing a spark through electric discharge brought about by a high voltage induced across the secondary winding 21b, a DC power supply source 24 for discharging, as an ion current i, those ions which are produced by the explosive combustion primed by the spark discharge of the spark plug 23, a resistor 25 connected in series to the DC power supply source 24 for converting the ion current i into a corresponding voltage signal, and an output terminal 26 for outputting the detected ion current value I in the form of the voltage signal mentioned above.
FIG. 7 is a waveform diagram showing a waveform of the ion current i. As can be seen from this figure, the ion current i increases as fire flame grows in succession to the explosion triggered by the spark discharge produced at the ignition plug 23 upon breakage of the primary current i.sub.1 in response to the ignition trigger pulse P.
Now, description will turn to the operation of the above-mentioned engine control apparatus shown in FIG. 5 by reference to FIGS. 6 and 7.
Let us consider ignition timing control for example. In this case, the engine control parameter setting circuit 31 sets the ignition timing with reference to the reference position which corresponds to a rising edge or a falling edge of the reference position signal T.theta. and determines the ignition timing so as to be optimal for the prevailing engine operation state D by consulting a data map or table, to thereby output as the control parameter Ta a control time or duration extending from the reference position to the ignition time point.
On the other hand, the misfire detecting circuit 32 determines the combustion state within each engine cylinder in each ignition cycle on the basis of the reference position signal T.theta. outputted from the angular position detector 1 and the detected ion current value vI outputted from the ion current detector 20 and generates a misfire detection signal C for the engine cylinder for which the detected ion current value I produced in the explosion stroke is lower than a predetermined reference level. The engine control parameter setting circuit 31 corrects, responsive to the misfire detection signal C inputted thereto, the control parameter Ta for the misfiring engine cylinder so that occurrence of misfire in that cylinder is suppressed. To this end, the ignition energy (or duration of the primary current i.sub.1 of the ignition coil) may be increased to thereby ensure ignition without fail. Further, in association with the fuel injection control, the injection period may be increased or decreased to enrich or thin the air/fuel mixture to thereby confirm whether misfire can be evaded by changing the air/fuel ratio. In case the misfire susceptibility can not be reduced or suppressed even by the correction of the control parameter Ta as mentioned above, fuel injection to the misfiring engine cylinder is stopped to thereby prevent the discharge of the uncombusted gas.
In general, when the power transistor 22 is turned off in response to an ignition trigger pulse P in an ignition cycle, a high voltage of negative polarity is applied across the electrodes of the spark plug 23 connected to the secondary winding 21b of a corresponding ignition coil 21, as a result of which an electric discharge takes place between the electrodes of that spark plug 23 to fire the gas mixture which then undergoes an explosive combustion. At this time, ions are produced within the engine cylinder due to ionization brought about by the explosive combustion. In that case, one of the electrodes of the spark plug 23 to which a bias voltage is applied from the DC power supply source 24 serves as an electrode for detecting the ion current i.
The ions produced through the aforementioned ionization within the engine cylinder are caused to migrate under the effect of the electric field of the bias voltage of positive polarity supplied from the DC power source 24, giving rise to a flow of the ion current i, which is then converted into the detection voltage I by the resistor 25 to be outputted from the output terminal 26. Thus, it is possible to make decision as to whether or not the gas mixture within the cylinder in each ignition cycle has been fired without fail by checking the level of the detected ion current signal I.
Usually, in the level decision of the detected ion current value I, a pulse obtained by shaping a part of waveform of the detected ion current value I which exceeds a threshold level, a peak-hold value, an integrated value or the like of the detected ion current signal I is employed. Further, since the detected ion current value I may vary in dependence on the engine operating state D, the reference level for comparison with the detected ion current value in the misfire decision may be appropriately modified or altered in dependence on the engine operating state D.
In conjunction with the ion current detection, it is however noted that there is a possibility that noise is generated upon driving of the ignition coil and superposed on the ion current I as detected. Consequently, when the ion current value I is retrieved at the timing for driving the ignition coil, the ion current level may undesirably be detected erroneously, leading to erroneous or false detection of misfiring or non-detection of the misfire which has really occurred. It goes without saying that such erroneous detection of the ion current level may result in unnecessary misfire suppression control or missing of the misfire suppression control at the time of occurrence of actual misfiring.
As will now be appreciated from the foregoing, the above-mentioned engine control apparatus in which a misfire detection signal C is generated through comparison of a detected ion current signal with a reference level without taking account of the timing for fetching the detected ion current value I suffers from the following problems. The reliability in misfire detection is degraded due to possible superposition of noise which generated upon driving the ignition coil, so the misfire evading or suppressing control may unnecessarily be performed or the misfire evading control can not be performed at all notwithstanding the fact that misfiring is actually taking place.